Thermography is an image-forming process including a heating step and hence includes photothermography in which the image-forming process includes image-wise exposure and direct thermal processes in which the image-forming process includes an image-wise heating step. In direct thermal printing a visible image pattern is produced by image-wise heating of a recording material.
U.S. Pat. No. 3,832,186 discloses in EXAMPLE 9 a heat developing-out photosensitive material obtained by coating a polyethylene terephthalate film support with a composition comprising polyvinyl butyral, silver benzotriazole, mercuric bromide, a benzoxazolidene thiohydantoin sensitising dye, ascorbic acid monomyristate as reducing agent and 3-mercapto-5-phenyl-4-phenyl-1,2,4-triazole and overcoating this composition with a vinyl chloride-vinyl acetate copolymer (weight ratio 85:15).
U.S. Pat. No. 3,933,508 discloses a heat developable light-sensitive material comprising, on a support, (1) an organic silver salt, (2) a light-sensitive silver halide or silver-halide prepared by reacting the organic silver salt with a halide, (3) a reducing agent, (4) a binder, and (5) an overcoat layer overlying components (1)–(4); said components (1)–(4) occurring in a single coated layer consisting essentially of a polymer having a refractive index greater than about 1.45 at 20° C., having heat resistance to temperatures greater than about 46° C., being colorless and soluble in organic solvents, and providing increased transparency for said overcoat layer. Examples of polymers used for the overcoat layer according to U.S. Pat. No. 3,933,508, which are preferably heat-resistant, colorless and soluble in solvents, are polyvinyl chloride, polyvinyl acetate, copolymers of vinyl chloride and vinyl acetate containing more than 50 mol % vinyl chloride but insufficient vinyl chloride to lower the heat resistance, polyvinyl butyral, polystyrene, polymethyl methacrylate, benzyl cellulose, ethyl cellulose, cellulose acetate butyrate, cellulose acetate, polyvinylidene chloride, polyvinyl pyrrolidone, cellulose propionate, polyvinyl formal, cellulose acetate phthalate, polycarbonates and cellulose acetate propionate. Moreover, gelatin, gelatin derivatives such as phthalated gelatin, acrylamide polymers, polyisobutylene, butadiene-styrene copolymers (no limitation on monomer proportions) and polyvinyl alcohol are preferred. EXAMPLES 5, 10, 12 and 18 exemplify an overcoat comprising a vinyl chloride-vinyl acetate copolymer (95 wt % vinyl chloride, 5wt % vinyl acetate).
GB-A 1,422,057 discloses a heat-developable photosensitive material comprising, coated on a support, a layer of (1) an organic silver salt, (2) a light-sensitive silver halide as grains of which at least 30 mole % is silver iodide, (3) a reducing agent and (4) a spectrally sensitising dye adsorbed on the surface of the silver halide grains, which sensitizing dye has an oxidation potential not exceeding 1.00 volt and difference in values between its oxidation potential and its reduction potential of at least 2.00 volts. EXAMPLE 4 discloses a silver benzotriazolate-containing heat-developable photosensitive layer with a top-coat of a vinyl chloride-vinyl acetate copolymer (85:15 by weight).
DE-A 24 39 460 discloses a thermally developable photosensitive material, comprising coated on a support, (a) an organic silver salt, (b) a catalytic quantity of a silver halide or photosensitive silver-halide prepared by reacting the organic silver salt with a halide, (c) a reducing agent and (d) a binder, and an overcoat, which comprises a polymer layer with kaolin dispersed therein. DE-A 24 39 460 discloses the following overcoat polymers: (a) copolyesters; (b) nylon compounds; (c) vinylidene dichloride copolymers; (d) ethylene-vinyl acetate-copolymers; (e) cellulose ethers; (f) polyethylene; (g) synthetic rubbers; (h) cellulose esters; (i) polyvinylesters; (j) polyacrylates and α-alkyl-polyacrylate esters; (k) high molecular weight polyethylene oxides of polyglycols with an average molecular weight of about 4000 to 1,000,000; (l) polyvinyl chloride and copolymers; (m) polyvinyl acetals; (n) polyformaldehydes; (o) polyurethanes; (p) polycarbonates; (q) polystyrenes; (r) gelatins and their derivatives; (s) polyvinyl alcohol; (t) naturally occurring colloids. EXAMPLE 4 discloses an overcoat layer comprising a vinyl chloride-vinyl acetate copolymer (95:5 by weight).
U.S. Pat. No. 4,197,131 discloses a light-sensitive composition comprising an intimate mixture of a substantially light-sensitive silver compound which upon reduction gives a visible change and sufficient of a silver halide to catalyse said reduction to give a visible change in those areas where the silver halide has been exposed to light and when the mixture is heated in the presence of a reducing agent, and as a yellow acutance dye a 1-alkyl-4-nitromethylene-quinolane, the alkyl substituent containing 1 to 4 carbon-atoms. The EXAMPLES disclose overcoating of the light-sensitive dispersion with a layer comprising a vinyl chloride-vinyl acetate copolymer available from Union Carbide under the name VYNS.
The EXAMPLES of U.S. Pat. No. 4,450,229 disclose the overcoating of a photothermographic element comprising a binder, light sensitive silver halide in catalytic proximity to a light insensitive silver source material in said element which contains a reducible source of silver ions, a reducing agent for silver ion and an effective latent image stabilizing of a particular class of compounds, with a layer comprising a vinyl acetate-vinyl chloride copolymer (80 wt % vinyl acetate, 20 wt % vinyl chloride).
U.S. Pat. No. 4,558,003 discloses an unexposed photothermographic article comprising a support, a silver containing layer comprising a binder, a silver compound having reducible silver ions, silver halide in catalytic proximity to said silver compound, and a reducing agent for silver ion, said binder comprising at least 25 percent by weight of a poly(vinyl acetal), said article being characterized by the presence of an amount of borate ion, a boric acid salt, or a boric acid, alone, or in combination with an isocyanate, sufficient to harden said binder. EXAMPLE 1 exemplifies a top coat of a poly(vinyl acetate)/poly(vinyl chloride)copolymer (80/20).
U.S. Pat. No. 4,460,681 discloses a color photothermographic imageable article comprising a substrate, a photothermographic emulsion layer, and organic solvent soluble barrier layer, a second photothermographic emulsion layer on a polymeric cover layer, wherein each of the photothermographic layers comprise a reducible silver source, photosensitive silver halide, a reducing agent for silver ion and solvent soluble binder, and further wherein each photothermographic layer is sensitized to a portion of the spectrum at least 60 nm different from the other photothermographic layer and each photothermographic layer contains a leuco dye which when oxidized forms a feasible dye having a maximum absorbance at least 60 nm different from that of the dye formed in the at least one other photosensitive layer, and the barrier layer is between said photothermographic layers and is impermeable to the solvent contained in the second photothermographic layer. Second layer “barrier” polymers may, according to U.S. Pat. No. 4,460,681, be maleic anhydride/vinyl methyl ether copolymers, polyvinylidene chloride (SARAN™), or polyvinylpyrrolidone, with maleic acid copolymers such as alkyl monoesters of poly(methyl vinyl ether/maleic acid) being preferred. The “barrier” polymer, which is the fourth layer and preferably contains the color reactants, is normally a methyl methacrylate polymer (preferably a hard polymer with a Tukon hardness of 20 or more), copolymer, or blend with other polymers or copolymers (e.g. copolymer with n-butyl acrylate, butyl methacrylate, and other acrylates such as acrylic acid, methacrylic acid, acrylic anhydride, and the like), polystyrene, or a combination of a polyvinyl chloride tripolymer with a butadiene-styrene copolymer. The preferred polymer is a hard methyl methacrylate homopolymer blended with soft methyl methacrylate copolymers. In EXAMPLE 5, a yellow/magenta barrier coating is disclosed of a butadiene/styrene copolymer and a polyvinyl chloride/acetate/alcohol) polymer (VAGH).
EP-A 0 536 955 discloses a heat developable photothermographic article comprising: (a) an image-receiving element comprising a polymeric image-receiving layer; and (b) strippably adhered to said image-receiving element, an imageable photothermographic element comprising a plurality of emulsion layers, each of which emulsion layers comprises a binder, a silver source material, and a leuco dye, and interposed between each pair of said emulsion layers a dye-permeable interlayer. EP-A 0 536 955 further discloses that the first interlayer must be impermeable to solvents to be used for applying the subsequent coating; however, the polymer for forming the first interlayer can be applied from any organic solvent. The polymer of the first interlayer is preferably a thermoplastic polymer. Homopolymers of vinyl chloride or copolymers of vinyl chloride, preferably having a glass transition temperature greater than 80° C., for example, a copolymer of vinyl chloride(96%) and vinyl acetate (4%), a blend of poly(vinyl chloride)(90%) and poly(vinyl acetate)(10%), can be used to form the first interlayer. EXAMPLE 1 exemplifies an interlayer comprising a terpolymer of vinyl chloride(83%), vinyl acetate(16%) and maleic acid(1%).
U.S. Pat. No. 4,942,115 discloses a thermally processable imaging element comprising a support bearing a thermally processable hydrophobic imaging layer and, on the side of the imaging layer away from the support, an overcoat layer comprising poly(silicic acid) and a hydrophilic monomer or polymer, wherein the element comprises a polymeric adhesion promoting layer between the overcoat and the imaging layer. U.S. Pat. No. 4,942,115 discloses that polymers that are useful in the polymeric adhesion promoting layer are: (1) terpolymers of 2-propenenitrile, 1,1-dichloroethene, and propenoic acid, such as disclosed in U.S. Pat. No. 3,271,345; and (2) terpolymers of 2-propenoic acid, methyl ester, 1,1-dichloroethene and itaconic acid as disclosed in, for example, U.S. Pat. No. 3,437,484, with a polymeric adhesion layer comprising poly(2-propenenitrile co-1,1-dichloroethene-co-2-propenoic acid) or poly(2-propenoic acid, methyl-ester-co-1,1dichloroethene-co-itaconic acid) being preferred and poly(2-propenenitrile co-1,1-dichloroethene-co-2-propenoic acid) being exemplified.
U.S. Pat. No. 6,352,819 discloses a high contrast black-and-white photothermographic material comprising a support having thereon: (a) a thermally developable, high contrast imaging layer(s) comprising a binder and in reactive association, photosensitive silver halide, a non-photosensitive source of reducible silver ions, a reducing composition for said non-photosensitive source reducible silver ions, and a high contrast agent, and (b) a barrier layer that is on the same side but farther from said support than said high contrast imaging layer(s), said barrier layer comprising a film-forming polymer and being impermeable to or reactive with any components that are diffusible from said image-forming layer(s) at a temperature greater than 80° C. U.S. Pat. No. 6,352,819 further discloses but does not claim a high contrast black-and-white thermographic material comprising a support having thereon: (a) a thermally developable and non-photosensitive, high contrast imaging layer(s) comprising a binder and in reactive association, a non-photosensitive source of reducible silver ions, a reducing composition for said non-photosensitive source reducible silver ions, and a high contrast agent, and (b) a barrier layer that is farther from the support than the high contrast imaging layer(s), the barrier layer comprising a film-forming polymer and being impermeable to or reactive with any components that are diffusible from said image-forming layer(s) at a temperature greater than 80° C. The term thermographic material is a generic term which includes photothermographic materials and the recitation of the high contrast thermographic material does not exclude the presence of photosensitive species such as photosensitive silver halide. There is therefore no specific disclosure in U.S. Pat. No. 6,352,819 concerning substantially light-insensitive thermographic materials and moreover U.S. Pat. No. 6,352,819 contains no examples devoted to substantially light-insensitive thermographic materials. Some particularly useful barrier materials, according to U.S. Pat. No. 6,352,819, include polyvinyl alcohol, a styrene polymer (including polymers of styrene derivatives), a vinyl halide polymer, a vinyl acetate polymer [such as polyvinyl acetate, poly(ethylene-co-vinyl acetate) or a copolymer of a vinyl halide and vinyl acetate], a polyvinyl pyrrolidone, a water-soluble or water-dispersible polyester, and gelatin (including deionised and acid processed gelatin) or a gelatin derivative (such as phthalated gelatin and carbamoylated gelatin, with the styrene polymers, vinyl acetate polymers and polyvinyl alcohol being preferred. EXAMPLE 12 discloses a barrier layer comprising poly(vinyl chloride-co-vinyl acetate), EXAMPLE 32 discloses a barrier layer comprising poly(styrene-co-acrylonitrile) (75:25 weight), EXAMPLE 33 discloses a barrier layer comprising U CAR VYNS poly(vinyl chloride-co-vinyl acetate) from Union Carbide and EXAMPLE 34 discloses a barrier layer comprising U CAR VAGH poly(vinyl chloride-co-acetate-co-vinyl alcohol) from Union Carbide.
It has been found that, in order to achieve a neutral image tone in substantially light-insensitive black and white monosheet thermographic materials, reducing agents and toning agents are required which diffuse to the surface of the material despite the presence of an outermost protective layer both during storage before printing and after printing and results, in extreme cases, in the user visually observing deposits of reducing agents, toning agents and reaction products of the imaging-forming process on the surface of the materials. Substantially light-insensitive black and white monosheet thermographic materials are therefore required which exhibit an acceptably neutral image tone, but do not exhibit the formation of such deposits of reducing agents, toning agents and reaction products of the imaging-forming process.